Aim: This in vitro study was conducted to evaluate the retention of zirconia and cast metal copings luted to implant abutments using different luting agents. Subjects and Methods: Implant analogs were embedded in acrylic resin blocks, and titanium abutments were torqued at 35 N/cm onto the implant analogs. The samples were divided into two groups. Both Group A and B samples were then randomly divided into four groups each comprising specimens – Group 1, Group 2, Group 3, and Group 4. Copings were then cemented with resin modifies glass ionomer cement (GIC), GIC, zinc phosphate cement, and noneugenol zinc oxide cement, respectively. The cemented copings were subjected to tensile dislodgment forces using a crosshead speed of 5 mm/min. Results for the retention of the copings were statistically analyzed using factorial analysis of variance test. Results: Between the two copings, higher mean retention is recorded in zirconia compared to metal, and the difference between them is found to be statistically significant (P < 0.001). Among the four types of cements, higher mean retention was recorded with resin-modified GIC followed by zinc phosphate and GICs. The least retention strength was recorded with noneugenol zinc oxide cement. Conclusion: Definitive cements are recommended for luting single-unit implant-supported metal restorations. The provisional cement was found to be least retentive and may not be suitable for luting single-unit implant-supported restorations, whether for a zirconia or for a cast metal restoration.

The provisional cement are least retentive and are not suitable for luting single-unit implant-supported restorations, whether for a zirconia or for a cast metal restoration. Therefore definitive cements are recommended for luting single-unit implant-supported metal restorations.

Introduction

Prosthodontically, driven implant placement has changed the options for restoring edentulous areas dramatically. Clinical decisions are not limited to the selection of the type of implant and the abutment; there is also the need to choose the type of cement.

Cement-retained restorations are passive superstructures, which easily load the implant axially and require traditional prosthetic techniques and laboratory skills for fabrication and yet give good control over esthetics and fewer appointments to restore the implant.

A study by Singer and Serfatyfound over a 3-year follow-up that the most cement failures occurred in implant-supported fixed partial dentures fabricated in the posterior region with short, 3-mm to 4-mm abutment heights.[1] Various authors have shown that the choice of cement material, amount of cement space or internal relief, occlusal forces, and type of luting agent can also affect the retentiveness of final restorations.[1],[2],[3],[4],[5],[6],[7],[8],[9]

Due to the large number of implant manufacturers with various components specific to each implant system, different provisional and definitive restorative options, and various cement materials, clinicians are left with limited information in the literature regarding the evaluation of the retentive capabilities of luting agents when used between metal components, such as cast metal and zirconia restorations cemented onto machined titanium implant abutments.

This study thus aims to compare the retentive strength of different classes of luting agents used to cement cast noble metal alloy and zirconia copings to implant abutments.

Subjects and Methods

The present study was conducted in the Department of Prosthodontics, Vokkaligara Sangha Dental College and Hospital, Bangalore.

Study method

Implant analogs were embedded in acrylic resin blocks, and titanium abutments were torqued onto the implant analogs. A 4-mm diameter hole was drilled at the end of the acrylic block to facilitate mounting the specimen on the tensile strength testing machine [Figure 1]a and [Figure 1]b. A single operator prepared all the eight resin blocks and eight implant analogs was embedded into it, and eight abutments were torqued at 35 N/cm. The abutments used in this study were 7.4 mm in height with 8° convergence angle and 4.2 mm in diameter. Then, the samples were divided into two groups. Group A with four samples (implant analog with abutment) and for each sample, eight copings of metal were casted, and Group B with four samples (implant analog and abutment) for which of each eight zirconia crowns were fabricated.{Figure 1}

While the wax pattern was made for the copings, an extension was made on the occlusal surface of each coping, parallel to the long axis of the tooth, serving as a connector to the tensile strength testing machine. A 4-mm diameter hole was drilled at the center of extension on the coping to facilitate mounting the specimen on the tensile strength testing machine [Figure 2]. Finally, the wax patterns were casted, trimmed, and finished. Moreover, the zirconia copings were designed and milled with cere 3 Sirona CAD-CAM unit.{Figure 2}

Both Group A and B samples were then randomly divided into four groups each comprising specimens – Group 1 to Group 4. Copings in Group A1 and B1 were cemented with RelyX™ U100 (3M ESPE); copings in Group A2 and B2 were cemented with glass ionomer cement (GIC) (GC gold label luting and lining cement); copings in Group A3 and B3 were cemented with zinc phosphate cement (DeTrey Zinc), and finally copings in Group A4 and B4 were cemented with zinc oxide noneugenol cement (Freegenol). Each coping in all the groups was sandblasted with 50 μm Al2O3 at 0.4 MPa before cementation. Cements were then mixed according to the manufacturer's instructions and were applied in a thin layer to the inner axial walls of the crown. Each restoration was seated on its respective abutment with firm finger pressure and then placed under a 10 kg weight for 5 min. Cementation was performed at an ambient temperature of 23°C ± 1°C. Excess cement was removed with an explorer.

The specimens were stored at room temperature for 24 h and then immersed in artificial saliva for 7 days. After 7 days, the specimens were stored in 100% humidity at 37°C for 1 h, thermal cycled 100 times between 5°C and 55°C with a dwell time of 10 s, and then stored in 100% humidity at 37°C for 6 days. Thermocycling was conducted to simulate thermal stresses and aging of the cemented copings. Specimens were dried and subjected to retention test.

Retention test

After 7 days of storage, the specimens were mounted on a tensile strength testing machine. They were subjected to tensile dislodgment forces using a crosshead speed of 5 mm/min [Figure 3] and [Figure 4]. The retentive force was determined as the maximum force required for coping removal and was recorded in Newton. Data were subjected to one-way analysis of variance (ANOVA). All hypothesis testing was conducted at the 95% level of confidence. Results were statistically analyzed using factorial ANOVA test.{Figure 3}{Figure 4}

Results

In this experiment, we have two factors influencing retention, namely, coping and cement. Coping is of two types – zirconia and metal. Cement is of four types – resin-modified GIC, GIC, zinc phosphate cement, and noneugenol zincoxide cement.

[Graph 1] depicts the mean tensile retention force (n) recorded in the Group A (metal copings) with four cements.[INLINE:1]

Group A1 metal copings were cemented with RelyX recorded a mean tensile retention force of 321.13 N. A mean tensile retention force of 184.63 N was recorded for Group A2 metal copings were cemented with glass ionomer. Group A3 metal copings were cemented with zinc phosphate recorded a mean tensile retention force of 204.88 N. Group A4 metal copings were cemented with zinc oxide recorded a mean tensile retention force of 97.63 N. Higher mean tensile retention force was recorded in Group A1 (RelyX) followed by Group A3 (zinc phosphate), Group A2 (glass ionomer), and Group A4 (zinc oxide).

[Graph 2] depicts the mean tensile retention force (n) recorded in the Group B (zirconia copings) with four cements.[INLINE:2]

Group B1 zirconia copings were cemented with RelyX recorded a mean tensile retention force of 349 N. A mean tensile retention force of 235 N was recorded for Group B2 zirconia copings cemented with glass ionomer. Group B3 zirconia copings were cemented with zinc phosphate recorded a mean tensile retention force of 238.25 N. Group B4 zirconia copings were cemented with zinc oxide recorded a mean tensile retention force of 152.88 N. Higher mean tensile retention force was recorded in Group B1 (RelyX) followed by Group B2 (glass ionomer), Group B3 (zinc phosphate), and Group B4 (zinc oxide).

[Table 1] shows that among the four types of cements, higher mean retention was recorded in resin cement followed by zinc phosphate and glass ionomer, respectively. The least retention strength was recorded in zinc oxide. The difference in mean retention among the cements was found to be statistically significant between resin cement and glass ionomer (P < 0.001), resin cement and zinc phosphate (P < 0.001), resin cement and zinc oxide (P < 0.001), glass ionomer and zinc oxide (P < 0.001), as well as zinc oxide and zinc phosphate (P < 0.001). No significant difference was observed between glass ionomer and zinc phosphate (P > 0.05).{Table 1}

Highest mean retention was recorded in resin cement with zirconia coping. Even with metal coping, highest mean retention was recorded in resin cement only. With the use of any of the four cements used in the study, higher mean retention is always found in zirconia coping. Lowest mean retention was recorded in zinc oxide with metal coping. Even with zirconia coping, lower mean retention was recorded in zinc oxide compared to the other cements.

[Table 2] shows the mean tensile retention force between the metal and zirconia copings. The difference between them was found to be statistically significant.{Table 2}

Discussion

Evaluation of cement-retained implant restorations should include a number of criteria based on specific physical properties and handling characteristics.[11],[12],[13] The ideal properties for this cement include (1) low viscosity for easy seating, (2) easy to mix, (3) extended working time, (4) short setting time, (5) insolubility in the mouth, (6) high shear, tensile, and compressive strength, (7) biocompatible, and (8) radiopaque.

In this study, the cements used included resin-modified glass ionomer, conventional glass ionomer, zinc oxide eugenol (ZOE), and zinc phosphate. Tarica et al.[14] surveyed US dental schools on cementation protocols for implant crowns and found that resin-modified GIC was the most frequently used luting agent for cementing implant restorations.

There is limited information in the literature regarding the comparison of retentive properties of zirconium copings and cast metal copings on implant abutments, using various luting cements.[15] One would expect that those cements formulated as permanent luting cements would be at the top of the retention list; however, Mansour et al. found that rank order of cement retentiveness differed when tested on implants rather than on natural teeth.[16]

In this study, zirconia and cast metal copings were sandblasted and subjected to thermocycling before testing for tensile strength. Studies reported that zirconia and cast metal restorations showed improved retentive forces, after sandblasting and thermocycling, which indicated good mechanical retention.[14] High mechanical strength would be related to the retentive force of the zirconia coping, as described by Li and White[17] who indicated that compressive strength was predictive of the retention of crowns. Lawson et al.[18] revealed that resin-based cements showed a correlation between flexural strength and crown retention, and Michalakis et al.[19] reported that for retrievability, the weakest retentive force cement should be recommended. However, as reported, the retentive force depends on the taper, surface roughness, and microgap.[20],[21]

In this study, higher mean retention was recorded with zirconia copings compared to cast metal copings. The reason may be due to the accuracy of fit of the CAD/CAM zirconia copings versus cast metal copings which were fabricated using the lost wax technique.[22]

In this study, the third-generation Cerec system, i.e., Cerec-3, was used to fabricate the zirconia copings, and studies have shown that this system was faster, easier, and had better fitting restorations and hence better retention compared to the other systems.[22]

Among the four types of cements used to lute zirconia copings, higher mean retention was recorded with resin-modified GIC. The resin cement adheres to titanium alloy by chelating metallic ions, but the retentive strength may be weakened by early water contact.[23] However, the new generation RelyX™ U100 used in this study is acidic and hydrophilic on application and becomes neutral and hydrophobic after setting. Therefore, it can resist water uptake better and remains more stable over time and shows zero solubility property. The results of thermocycling stress tests on self-adhesive resin cements show that RelyX™ U100 cement remains intact, while the conventional resin cement showed obvious cracking.

The higher mean retention of resin cement is also due to the presence of additional monomer content which gives a tensile strength of 99Mpa to the set cement when compared to the conventional resin cement and enhances self-bonding to zirconia. During setting, self-adhesive resin cements typically undergo a change in pH from acidic (pH 2–3) to less acidic (pH 5–6). The early acidity of the cement allows it to etch and adhere directly to the zirconia.[24] Studies have shown significantly higher bond strength to Lava™ zirconia, e.max, and Paradigm™ C than all other cements (P < 0.05) when cemented with RelyX cements.[25] Another reason could be the mechanical bond between the resin cement and titanium alloy. Studies have revealed that a combination of resin cement and titanium alloy has higher bond strength compared to other alloys.[19]

The highest mean retention was found in resin cement followed by GIC and zinc phosphate. The mean retention of both the cements, when considered along with standard deviation, was found to be similar in this study. It may be due to the formation of a similar cohesive bond between the cement and the zirconia and also between cement and titanium alloy.[26]

Another reason for the decrease in bond strength compared to resin cement may be due to degradation of the luting cement itself[27] and the hydrolytic effect of water at the luting cement/zirconia interface.[28] Studies have revealed that the solubility of GIC is more than that of zinc phosphate cement and is very susceptible to early water contact and desiccation,[21] which can dramatically reduce the mechanical properties of the cement. It could be the mismatch between the coefficient of thermal expansion of the bonded specimens which result in hoop stress during thermocycling.[28]

The least retention was recorded in noneugenol ZOE. In several studies, this provisional cement has shown to have a decreased retention, especially after thermocycling.[29]

In a study on titanium copings over short ITI abutments, the mean retentive values of zinc phosphate cement were comparable to resin cement, but the comparative retentive strength between zinc phosphate and resin cement was higher than in the present study. A combination of resin cement and titanium alloy was reported to have higher bond strength compared to other alloys.[16]

In addition, variations in chemical composition, wetting capacity, viscosity, and mechanical properties for each luting cement could be responsible for variations in the bonding capacity of zirconia ceramics.[7],[20],[30],[31],[32]

Among the four types of cements used to lute cast metal copings, highest mean retention was recorded in resin-modified GIC due to its adhesive nature and chemical reaction. The composition of this cement differs from conventional GIC formulations in several key ways. First, in addition to ion-releasing glasses, conventional silanated composite resin glasses are included which impart greater mechanical strength. Second, the phosphoric acid groups are connected to the reactive carbon double bonds of the acrylate component through a carbon backbone. Using this kind of multifunctional monomer more fully integrates the polyacrylate salt into the interpenetrating network. These materials typically contain both chemical and light-induced radical polymerization reactions. The reactive phosphoric acid esters are negatively charged, and the bonding mechanism is through interaction with the positively charged base metal alloy.[25]

Zinc phosphate showed better retention than GIC. Zinc phosphate cements provide casting retention by micromechanical interlocking into the casting and the abutment surface irregularities.[33] Therefore, the utilization of surface irregularities for retention of dental restorations depends on the compressive strength not only of the cement but also of the adjacent metal material. This concept has important implications in cement-retained prostheses. Studies have shown that, cements which provide casting retention mainly by mechanical interlocking also show roughness and greater retention than adhesive cements.[27]

Surprisingly, 37.5% cast metal copings cemented with GIC had the lowest retention among the definitive luting agents, which were comparable to the provisional cements. This finding is consistent with a study that revealed GIC showed significantly lower retention than zinc phosphate and resin cements and similar retention to noneugenol ZOE.[4] It could be due to the solubility property of the GIC. GIC shows initial solubility as high as 0.4% due to leaching of intermediate products. Part of the absorbed water acted as a plasticizer, inducing a decrease in strength.

GIC is a polyacrylic acid-based cement bonds to clean oxidized metals and slightly to nonoxidized metals.

Lowest mean retention was recorded in noneugenol zinc oxide cement luted to metal copings, and this cement showed tensile strength considerably less compared to those cemented with zirconia copings. This may be due to the difference in coefficient of thermal expansion between the cement and the base metal alloy. Noneugenol zinc oxide cement has a very low coefficient of thermal expansion.

In one of the study, zinc phosphate, and ZOE had similar retentive strength.[34] The results obtained with the present study do not indicate the same which may be due to the usage of different cements and thermal cycles. Long-term thermal cycling has been shown to reduce the retentive strength of luting agents.[4],[34],[35] Squier et al.[36] reported a higher retentive value for glass ionomer than zinc phosphate which differs with the current study. This discrepancy may be related to sandblasting and surface roughness.

In this study, the type of cement was the major retentive factor for consideration apart from standardizing the taper, surface area, height, and marginal accuracy.[14] Therefore, a comparison with other studies may not be a feasible idea because only cement has been studied as a retentive factor.

In this study, the resin cement and GIC completely adhered to the intaglio surface of the copings. However, the provisional cement adhered to both coping surface and abutment surfaces and was easily removed showing least adhesive properties.

The results of this study suggested that the tested definitive cements are not desirable if retrievability of restorations is sought, especially if the clinician needs to remove the crown for hygiene, design modifications, fracture repairs, abutment screw tightening, and in case of single implant restorations which would be prospective implant to support implant supported bridges. The dislodgment force for an implant with 4-mm diameter and 6-mm height directly out of the socket along its long axis was about 290 N,[23] and the mean retention recorded with the definitive cements in this study is all above 290 N and an early attempt to retrieve the crown might lead to loss of stability and also failure of implant.

The passive fit of the cement-retained implant restorations allows for better assessment of the role of luting agents to crown retention. The force that is used to retrieve the implant-supported restoration is high impact and of short duration. On the other hand, in this study, a monotonic static test was used. The failures of the restorations result in several comparative small dynamic loadings, and the axial dislodging of cemented implant crowns is a rather seldom clinical event. Cement behavior under fatigue loading may be different compared with static loading. The results of this in vitro study should be interpreted with caution and confirmed with clinical studies. A thorough understanding of the cement material being used to properly use the cement to the best of its design characteristics is recommended.

Conclusion

Within the limitations of this study, following conclusions were drawn; highest mean retention was recorded for zirconia copings against the cast metal coping. Wherein, highest mean retention was recorded with resin-modified GIC followed by GIC and zinc phosphate cement.

For cast metal copings, highest mean retention was recorded with resin-modified GIC followed by zinc phosphate cement and GIC. Least mean retention was recorded with noneugenol zinc oxide cement for zirconia and cast metal copings. Cast metal copings cemented with GIC had the lowest mean retention among the definitive luting agents, which was comparable to the provisional cement and noneugenol zinc oxide.